Diphthong is a word that comes from the Latin diphthongus, which in turn derives from Greek and refers to the meeting of several different words that are expressed in the same syllable.
It should be noted that for there to be a diphthong in a sentence, one of them must be weak, for example in Spanish the vowels without force are i and u. For the diphthong to form, two different weak vowels must be together, for example, "diurnal" or by a strong vowel and a weak vowel, however in this case the vowel that is weak cannot be a tonic, because then it would no longer be a diphthong..
If there is a weak vowel next to another that is strong, the diphthong in that case receives the name of increasing and that happens when the second vowel constitutes the nucleus of the syllable, it can also be decreasing and this is when the nucleus is in the first vowel.
Apart from these two types of diphthongs, there is another that is rare and which is called a homogeneous diphthong. With respect to this case, it can be determined that this type of diphthong exists when a word begins and ends with a closed vowel.
It should be borne in mind that in grammatical matters there are various terms that often confuse people, as is the case with the hiatus. These are the union of two vowels that are pronounced in different syllables and that do not form a diphthong. More precisely, they are usually composed of a closed vowel and another strong vowel or two strong ones.
